Exploring skeletal asymmetry and indicators of developmental stress in a South African sample

Abstract

Biological anthropologists have shown great interest in understanding health and disease and its correlation to skeletal asymmetry. Fluctuating asymmetry, which is defined as the random deviation from perfect symmetry resulting in inequality in size or shape of bilateral traits, is often used to understand this correlation. Literature has shown that fluctuating asymmetry results from developmental instabilities and could be indicative of developmental stressors and an individual’s quality of life. Skeletal asymmetry and its correlation to different developmental stressors provide invaluable information regarding the interpretation of skeletal variation often observed among individuals. The understanding of human skeletal variation has many applications, ranging from forensic skeletal identification to facial surgery. While traditional methods for studying facial and dental asymmetry have been used in the past, the methods can be methodologically challenging, and not always practical in clinical settings. As such, virtual biological anthropology has become an increasingly popular alternative. Among the imaging modalities, micro-focus X-ray computed tomography (micro-XCT) is often considered as the gold standard, because of its non-invasive and non-destructive properties, as well as its remarkably high resolution and its consistency compared to other micro-XCT systems. Micro-XCT imaging has thus proven to be extremely useful for better evaluation of facial structures, with detailed images that can assist in identifying and quantifying facial asymmetry, especially when employed in conjunction with geometric morphometrics. Therefore, this study aimed to assess facial asymmetry in a South African population using micro-XCT and further explore the link between asymmetry and developmental stress. One hundred and fifteen individuals (59 black South Africans and 56 white South Africans, with 57 females and 58 males) and their associated micro-XCT scans, sourced from the Pretoria Bone Collection (University of Pretoria) were analysed to evaluate facial asymmetry. Anatomical landmarks were employed to take a series of cranial measurements and collect 3D coordinate data for geometric morphometric analysis. The measurements and extraction of 3D coordinate data were performed on 3D models virtually extracted from micro-XCT scans of crania of the same individual. Once collected, fluctuating asymmetry indices were calculated. The entire skeleton of the individuals was assessed for pathological lesions linked to nutritional disease and to assess the overall link to developmental stress of the individuals. The location and number of lesions was recorded for each individual. Statistical analyses were employed to assess intra- and inter-observer reliability for landmarks, measurements, and pathology analysis; to examine any significant differences between the left and right distances and shapes for measurements and geometric morphometric analysis, respectively; and finally, to evaluate the correlation between the presence of pathological lesions and the degree of asymmetry expressed in an individual. This study showed that the orbits, nasion and temporal regions expressed a high magnitude of asymmetry, particularly in black South African females. However, no link was found between asymmetry and signs of developmental stress. Thus, more research can be done to understand how, and when developmental stressors may influence skeletal asymmetry.